WASHINGTON — The only vote so far in this Congress on gun restrictions would impose tougher penalties for firearms trafficking, a measure that wouldn’t have prevented the Dec. 14 shootings at a Connecticut elementary school.

The Newtown victims were killed with a legally purchased gun.

The narrow measure approved by the Senate Judiciary Committee on March 7, increasing penalties for laws already on the books, is the lone piece of legislation to curb gun violence with clear bipartisan support in Congress. Efforts to push weapons bans have been hobbled, and the chairman of the House panel handling firearms legislation opposes expanding background checks for gun buyers.

Lawmakers who support restrictions say their agenda of banning assault weapons and high-capacity ammunition magazines such as those used to kill 20 students and six school employees in Newtown is at risk of being significantly watered down.

“This battle is going to be incremental,” said Sen. Richard Blumenthal, D-Conn., who has pressed for gun restrictions since Dec. 14, when he waited with parents in a firehouse near Sandy Hook Elementary School for word of their children. “It’s going to be achieved in steps and, tragically and very unfortunately, these shootings continue.”

The growing complexity of the debate on Capitol Hill reflects the influence of the National Rifle Association, a Second Amendment advocacy group that spent $16.6 million on television advertising in the 2012 election cycle, including $12 million against President Barack Obama, according to the Washington-based Center for Responsive Politics.

The gun lobby opposes any restrictions on ownership of firearms and ammunition, and wants to thwart an expansion of background checks.

Advocates for stricter gun laws had counted on public outrage over the Newtown shootings and Obama’s support to end a two-decade drought in major U.S. gun legislation. The 1994 ban on assault weapons expired in 2004.

“The calls have been coming in as if this is some kind of wild-eyed scheme — it is not,” Sen. Dianne Feinstein, D-Calif., who sponsored the assault weapons ban, said at the March 7 meeting, citing polls showing Americans support gun restrictions. “It’s been a very hard road.”

Almost three months after Newtown, many in Congress are reluctant to support measures restricting gun purchases. While much of the opposition is in the Republican-led House, Democrats from pro-gun states such as North Dakota and Montana may make it difficult for Senate Democratic leaders to muster the 60 votes needed to advance major legislation. Republicans say Congress should enhance and enforce current gun laws.

“I have a hard time explaining to my constituents back home how passing more laws that will go unenforced makes them any safer,” Sen. John Cornyn, R-Texas, said at the March 7 Senate panel meeting.

In Maryland, where Democratic Gov. Martin O’Malley’s party controls the legislature, lawmakers may give final passage to some of the nation’s strictest gun-control laws. The issue has drawn opposition from Accokeek, Md.-based Beretta USA Corp.

The Maryland Senate approved legislation on Feb. 28 that would ban assault rifles and high-capacity magazines, keep guns from the mentally ill and impose new licensing rules for handguns, including training and fingerprinting requirements for permit applicants. It is pending in the House of Delegates.

State legislatures are considering more than 600 gun control bills and about 540 measures broadening gun rights, according to a March 5 analysis by the San Francisco-based Law Center to Prevent Gun Violence.

Polling shows more than half of Americans support laws restricting access to assault weapons and high-capacity magazines. A Quinnipiac University poll conducted Feb. 27 to March 4 found 88 percent of respondents, including 83 percent of Republicans, support background checks of gun buyers.

Families of gun-violence victims have delivered emotional pleas before Congress for action, and law enforcement officials have testified in favor of tougher laws. Still, backers of new restrictions express doubts that much will get done this year.

“It’s politics: the congressional leaders don’t have the courage to do what’s morally right,” said Po Murray of Newtown Action Alliance, a group founded to press for stricter gun laws after the Newtown shootings.

“They’re caught up in that box of what needs to be done to represent the gun lobby,” said Murray, citing the political influence of the NRA.

During a recent Senate Judiciary hearing, only one Republican and four Democratic lawmakers were in attendance for the tearful plea for an assault weapons ban by Neil Heslin, the father of a six-year-old boy killed on Dec. 14 and the only Newtown parent to testify before Congress.

In January, Obama unveiled the most ambitious agenda of gun restrictions in decades, a $500 million package including mandatory background checks for all firearms purchases and bans on high-capacity magazines and assault weapons.

His spokesman, Jay Carney, said on March 7 that the president is “encouraged” by progress Congress is making on guns and immigration legislation. With Democrats seeing a ban on high-capacity ammunition magazines as difficult to achieve as an assault weapons ban, background checks are the remaining focus of Obama’s push.

In the Senate, New York Democrat Chuck Schumer is seeking the support of Illinois Republican Mark Kirk and Democrats from pro-gun states, including Joe Manchin of West Virginia, in negotiations as he advances his background-check bill.

On March 6, former representative Gabrielle Giffords and her husband, retired astronaut Mark Kelly, urged Congress to act on background checks by returning to the Safeway parking lot in Tucson, Ariz., where in 2011 she was injured in a shooting that killed six people, including a federal judge and a 9-year- old girl.

Kelly cited the 1.7 million people who failed background checks since 1999. Many were probably able to obtain weapons through gun shows or the Internet, he said.

“The thing we can do right now, today, the thing that has momentum, the bill that can get through the Senate and hopefully the House is a universal background check bill,” Kelly said. He and Giffords founded a gun-control advocacy group in January called Americans for Responsible Solutions.

Whether such a bill will advance in the House is uncertain. In 1999, the NRA’s chief executive officer testified before Congress in favor of ending the exception to federal background checks for private gun shows. The NRA now opposes ending this exception as well as requiring background checks for private sales between non-family members.

The main objection of the Fairfax, Va.-based group, which claims more than 4 million members, is a requirement that licensed gun dealers keep a record of the screenings. The NRA says this could lead to a national registry of gun owners. NRA President David Keene recently said a requirement for universal background checks would be a burden for law-abiding gun owners.

Democrats in Congress say there is no plan to create a federal registry, already prohibited by law, and that sales records are necessary to track weapons found at crime scenes.

“How does a background check create a registry if there is no central record-keeping by the Department of Justice?” Blumenthal said.

House Judiciary Chairman Bob Goodlatte, the Virginia Republican who presides over gun legislation in the House, said on Feb. 27 that he opposes requiring universal background checks and that he doesn’t plan to take up the issue.

There’s been no discussion of the matter in Republican conference meetings, said Rep. Carolyn McCarthy of New York, a Democrat who favors gun restrictions and has been pushing for them with her House colleagues.

Democrats say their best chance for expanded background checks is if Senate support persuades House Republican leaders to advance the bill. Advocates say a coalition of as many as 40 House Republicans, from the Northeast and states such as Virginia that have experienced recent shooting sprees, may back an expansion.

Representative Mike Thompson, a California Democrat who was chairman of a Democratic panel on gun violence, supports that view. On a ban on assault weapons and high-capacity magazines, though, he suggested that Senate Judiciary legislation will probably fail after a vote by the panel. He applauded Feinstein’s efforts.

“The fact that she gets it out of committee still heightens awareness to the issue of gun violence,” he said.

Meanwhile, Republicans are backing measures that don’t restrict weapons purchases. On March 6, Republican Sens. Lindsey Graham of South Carolina and Jeff Flake of Arizona introduced a bill to expand the type of mental health records fed into the federal background data base used by gun sellers.

The trafficking measure approved by the Senate panel would set prison sentences of as much as 20 years for straw purchasers, or those who knowingly buy a weapon for an individual prohibited from owning it.

The measure wouldn’t increase the number of gun transactions subject to screening. The Senate Judiciary Committee is scheduled to reconvene for votes on the assault weapons ban and background checks Tuesday.